Using two model systems investigations of melatonin's possible intracellular action were assessed. In vitro, MCF-7 human breast cancer cells were chosen and in vivo, uteri from ovariectomized hamsters were used. Melatonin had previously been shown by us to have an acute effect on estrogen receptors in both systems, and one hypothesized modification of estrogen receptors is their phosphorylation to an active form. This phosphorylation might be regulated by cAMP dependent protein kinase, suggesting an effect of melatonin on cAMP concentrations, or cGMP concentrations, or on cAMP-dependent protein kinase. We have investigated melatonin's effect on these three parameters in these two model systems.